Technical Field
The present invention relates to a fuel assembly and, in particular, to a fuel assembly suitable for plying to a boiling water nuclear reactor.
Background Art
A plurality of fuel assemblies are loaded in a core of a boiling water nuclear reactor. Each fuel assembly has a fuel bundle disposed in a square tubular channel box. Each fuel bundle has a plurality of fuel rods in which a plurality of fuel pellets containing uranium are disposed, an upper tie plate which supports upper end portions of the fuel rods, a lower tie plate which supports lower end portions of the fuel rods, and a plurality of spacers, each of which maintains space among the fuel rods. One example of the fuel spacer is disclosed in Japanese Patent Laid-Open No. 62(1987)-105082. In general, these fuel spacers are disposed at predetermined intervals in the axial direction of the fuel assembly for the purpose of holding the fuel rods for preventing the fuel rods from bending or the like. In the boiling water nuclear reactor, cooling water boils in each fuel assembly loaded in the core of the reactor, and a part of the cooling water is converted into steam, and a gas-liquid two-phase flow containing the cooling water and steam goes up in the fuel assembly. The steam contained in the gas-liquid two-phase flow is separated from the cooling water by a steam-water separator and a steam dryer in the nuclear reactor. The separated steam is supplied from the nuclear reactor to a turbine connected to an electric generator.
If a surface of the fuel rod is in a state of being covered with a liquid film of the cooling water in the core, heat removal from the fuel rod is soundly achieved. However, if the surface of the fuel rod is in a state of being in contact with steam constantly, heat removal from the fuel rod is carried out poorly, and as a result, the surface temperature of the fuel rod is increased. In order to prevent this increase in the surface temperature, it is necessary to bring the surface of the fuel rod to a state where the liquid film is present on the surface constantly.
In an upper portion of the fuel assembly where the ratio of steam contained in the gas-liquid two-phase flow is increased, the liquid film is formed on each surface of the fuel rods, and a phenomenon called an annular-dispersed flow in which steam and liquid droplets are present occurs between the fuel rods. The gas-liquid two-phase flow goes up between the fuel rods while the adhesion of the liquid droplets in the steam to the liquid film on the surface of the fuel rod, and the scattering of the liquid droplets into the steam from the liquid film are repeated. In order to maintain the liquid film formed on the surface of the fuel rod, it is only necessary that the liquid droplets can actively adhere to the liquid film from the steam. As a result, it is possible to improve the thermal margin of the fuel assembly.
The fuel spacers have a function of disturbing the flow of the gas-liquid two-phase flow going up a cooling water path formed between the fuel rods, and therefore, an effect of allowing the liquid droplets in the steam to adhere to the liquid film on the surface of the fuel rod is enhanced. However, the mechanism of disturbing the gas-liquid two-phase flow in the cooling water path by the fuel spacer causes a large pressure loss. This large pressure loss causes a decrease in the flow rate of the cooling water going up in the fuel assembly. The pressure loss of the fuel spacer is related to a projected area of the fuel spacer closing the cooling water path. Due to this, the thermal margin of the fuel assembly was tried to be improved by disturbing the flow of the gas-liquid two-phase flow in the fuel assembly while decreasing the pressure loss by decreasing the projected area of the fuel spacer to the cooling water path.
Further, in the fuel assembly in which a part of the plurality of fuel rods are substituted with partial length fuel rods, the pressure loss of the fuel assembly is decreased by the use of the partial length fuel rods. By using the partial length fuel rods, a space where fuel rods are not present is formed on an upper side of the upper ends of the partial length fuel rods in the fuel assembly to decrease the pressure loss, and thus, the cooling water easily flows in the fuel assembly. However, the adoption of the partial length fuel rods increases the amount of liquid droplets flowing along with steam from the fuel assembly.
In the fuel assembly described in Japanese Patent Laid-Open No. 2010-145232, partial length fuel rods are disposed in an outermost layer of a fuel rod array and an inner region which is a region on the inner side excluding a second layer from the inner surface of a channel box, and a partial fuel spacer having a size smaller than reference fuel spacers in the direction perpendicular to the center axis of the fuel assembly is disposed on the upper side of the upper ends of the partial length fuel rods in an inner region where these partial length fuel rods are disposed. The reference fuel spacers support all the fuel rods in the fuel assembly. On the other hand, the partial fuel spacer supports fuel rods, which are a part of all the fuel rods in the fuel assembly and are disposed in the inner region. The partial fuel spacer allows liquid droplets present in a space on the upper side of the partial length fuel rods to adhere to a liquid film on the surface of the fuel rod to decrease the amount of liquid droplets flowing out to the outside of the fuel assembly. Further, the increase in the pressure loss of the fuel assembly is minimized by disposing the partial fuel spacer.
In the fuel assembly described in Japanese Patent Laid-Open No. 2001-318182, it is intended to increase the dry out margin of fuel rods and to decrease the pressure loss of the fuel assembly by supporting a plurality of fuel rods by two types of fuel spacers. These two types of fuel spacers are a fuel spacer having a low pressure loss and a fuel spacer having a large mixing effect. The latter fuel spacer includes blades for swirling cooling water attached to each grid plate in the vicinity of each intersection of the orthogonally provided grid plates. The former fuel spacer is not provided with such blades and achieves the decrease of a pressure loss. The fuel spacer having a low pressure loss and the fuel spacer having a large mixing effect are alternately arranged in the axial direction of the fuel assembly.
Further, a water gap region (a region on the outer side of a channel box) in which saturated water is present is formed between the adjacent fuel assemblies loaded in the core, and a gas-liquid two-phase flow is present in the fuel assembly. Therefore, neutrons generated by nuclear fission of a fissile material in the fuel rods are easily moderated in the water gap region. Many of the neutrons causing nuclear fission of U-235 are supplied to the inside of the fuel assembly from the water gap region. As a result, the power in an outer peripheral portion in a cross section of the fuel assembly (a cross section in the direction perpendicular to the center axis of the fuel assembly) is increased. Since a thermal margin in this outer peripheral portion is not enough, in a fuel assembly of a general boiling water nuclear reactor, the uranium enrichment in the fuel rods disposed in the outer peripheral portion is lower than that in the fuel rods disposed in the inner side. In the cross section of each fuel assembly loaded in the core of a general boiling water nuclear reactor, the ratio of the average uranium enrichment in the outer peripheral portion in the cross section thereof to the average uranium enrichment in the cross section thereof is 0.9 or less. On the other hand, if the thermal margin of the fuel assembly can be ensured, the enrichment of the fissile material in the fuel assembly can be increased, and thus, the inventory of the fissile material can be increased.